1. Field of the Invention
The present invention relates to a communication terminal which is connectable to a plurality of base stations, and an encoding rate reduction method in the communication terminal.
2. Description of the Related Art
In recent years, in a cellular mobile communication network, with multimedia of information, in order to implement large-capacity data transmission, a technique which uses wireless communication in a high frequency band to implement a high transmission rate (communication speed) is actively studied.
In the cellular mobile communication network, for example, there are an LTE (Long Term Evolution) protocol, a W-CDMA (Wideband Code Division Multiple Access) protocol or a CDMA2000 protocol classified into 3G (3rd Generation), an HSPA (High Speed Packet Access) protocol classified into 3.5G, and a GSM (Registered Trademark) (Global System for Mobile Communication) protocol classified into 2G (2nd Generation). In the cellular mobile communication network, a plurality of mobile communication networks are mixedly used.
When one wireless communication mobile terminal (hereinafter, referred to as a “mobile terminal”) is connectable to a plurality of different mobile communication networks, handover (heterogeneous network handover) between cells of different kinds of mobile communication networks may occur depending on quality of a communication channel to be used by the mobile terminal. The heterogeneous network handover is, for example, handover from a mobile communication network of LTE, in which communication is possible at a high transmission rate, to a mobile communication network of an HSPA protocol or a CDMA-EVDO (Evolution Data Only) protocol, in which communication is performed at a transmission rate lower than that of LTE, and is called Inter-RAT (Radio Access Technology) handover. Hereinafter, the heterogeneous network handover is referred to as “Inter-RAT handover” or “Inter-RAT HO”.
For example, it is assumed that a mobile terminal which is transmitting image data for videoconference performs Inter-RAT handover from a mobile communication network of LTE, in which communication is possible at a high transmission rate, to a mobile communication network of an HSPA protocol, in which communication is performed at a transmission rate lower than that of LTE. In this case, a transmission rate which is usable for transmission of image data in the mobile terminal decreases from an average transmission rate (several Mbps to tens of Mbps, and for example, 10 Mbps) in the mobile communication network of LTE to an average transmission rate (hundreds of kbps to several Mbps, and for example, 1 Mbps) in the mobile communication network of HSPA.
In order to reduce flickering of a display image on a reception side with the decrease of the transmission rate which is usable for transmission of image data in the mobile terminal, for example, a transmission device and a transmission method shown in JP-A-2011-82837 are known.
With a decrease in wireless communication quality (for example, in JP-A-2011-82837, an error rate), when a wireless transmission rate which is usable for transmission of image data decreases, it is considered that the amount of data packets of encoded image data to be stored in a buffer increases while a high image encoding rate is maintained on a transmission side, a buffer overflow occurs, data stored in the buffer is not transmitted, and an image error occurs. For this reason, the transmission device shown in JP-A-2011-82837 immediately lowers an image encoding rate when the error rate as wireless communication quality is lower than a predetermined threshold value.